Effects of anterior cruciate ligament resection and total knee arthroplasty on lower limb alignment and tibiofemoral rotation

BACKGROUND: The anterior cruciate ligament (ACL) is a key structure that restricts excessive anterior translation of the tibia and maintains rotational stability. This study aims to dynamically and quantitatively assess the direct effects of ACL resection and total knee arthroplasty (TKA) on lower limb alignment and tibiofemoral joint rotation under passive, unloaded conditions and in the pathological state of osteoarthritis. METHODS: This retrospective study included 110 patients with varus knee osteoarthritis who underwent Mako for robotic-assisted TKA. Dynamic intraoperative measurements of the lower limb mechanical axis (flexion angle at maximum extension; varus angle) and tibiofemoral rotation at multiple flexion angles (min to max flexion) were recorded at three intervals: pre-ACL resection, post-ACL resection and post-TKA. The variation in rotation amplitude was calculated for successive flexion intervals. RESULTS: Post-ACL resection, a significant decrease in the flexion angle at the maximum extension and varus angle (both p < 0.001) was observed, which was accompanied by increased tibial internal rotation at flexion angles of 60° and above (p < 0.05). Varus alignment was successfully corrected following TKA (p < 0.001). However, tibiofemoral rotational kinematics were significantly modified: the amplitude of internal rotation decreased in the initial flexion arc (min to 30°) but increased in flexion intervals beyond 30° (all p < 0.01). CONCLUSIONS: Under passive, unloaded osteoarthritic conditions, ACL resection immediately alters lower limb alignment and increases tibial internal rotation. Although TKA restores coronal alignment, it does not fully restore passive rotational kinematics; whether this affects active weight‑bearing function remains unknown. Accordingly, we hypothesize that rotational stability is as critical as limb alignment in TKA, pending validation with patient-reported outcome measures, functional scores and postoperative follow-up.